Joystick controllers are used in a wide variety of control applications. One example is in the control of an excavator bucket. Movement of the bucket is actuated by hydraulic pistons, which are controlled by electrical signals provided by the joystick. The joystick has a lever, which is mounted for pivotal movement relative to a fixed body. Movement of the lever in one direction, by pivoting the lever about a first axis, controls the raising and lowering of the bucket. Movement of the lever in a second direction, by pivoting about a second axis (usually orthogonal to the first axis), controls orientation of the bucket (i.e. moves the bucket by turning it to the left or the right).
When excavating earth it is frequently required to place the bucket controls into a so-called float configuration, in which the raising and lowering controls are over-ridden so that the bucket drops under its own weight to the ground. In the float configuration, the bucket stays at the ground and rests on the terrain. There is no hydraulic influence over its vertical position. However, it is still desirable that an operator of the excavator should be able to control the left-right orientation of the bucket using the joystick while leaving the bucket level free to move with terrain change. A float configuration can be achieved with a joystick that has a lock facility to prevent movement of the joystick lever in the direction that controls vertical movement of the bucket. When the joystick is locked, the vertical controls are over-ridden.
One such joystick controller has been described in GB 2,313,175. This joystick has a bush that can slide up and down a shaft of the lever and is biased by a spring against a cradle mounted on the joystick body. The cradle has a recess, which engages a shoulder of the bush when the joystick lever has been displaced by a certain angle about a first axis (say the x-axis), to hold the lever at that angle.
A problem with this arrangement is that, when in the locked position, pivotal movement of the joystick in the other direction about the orthogonal y-axis can only be achieved by a corresponding pivotal movement of the bush and cradle. This means that the cradle must be mounted to the joystick body in such a way that it is allowed to pivot about the y-axis. Furthermore, this joystick uses a gimbal arrangement by which the joystick lever is mounted to the body for pivotal movement. Nowadays it is often preferable to use a ball and socket arrangement for mounting the joystick lever.
Many known joystick controllers include a return-to-centre arrangement, so that when the lever is displaced and subsequently released it is biased back towards a central position. The degree of biasing force also provides a tactile feedback by which the operator can sense the extent of displacement. However, there are many applications where it may be required to keep the joystick at a displacement in one (e.g. x) direction. It is a problem to achieve this while still retaining the return-to-centre feature in the orthogonal y direction.